1.Field of the Invention
This invention relates to an apparatus for optically recording or reproducing information and, more particularly, to an optical information recording and reproducing apparatus for preventing overwrite recording, i.e., preventing new information from being recorded over information already recorded.
2. Description of the Related Art
As conventional information recording systems for recording information in the form of digital signals, magnetic recording systems, optical recording systems and magneto-optical recording systems are known. Optical recording systems will be described below by way of example. Various types of recording media in the form of a disk, a card, a tape and the like are known as a medium on which information is optically recorded or from which information is read out. It is believed that a card-like recording medium (hereinafter referred to as an "optical card") will be extensively used because it can be manufactured easily and has good portability and accessibility.
Various optical information recording and reproducing apparatuses have been proposed for use with such an optical card. In each such apparatus heretofore proposed, information is recorded or reproduced while automatic tracking control and automatic focusing control are being continuously performed. Information is recorded on a recording medium in this kind of apparatus by scanning information tracks with a light beam modulated in accordance with the recorded information and condensed to form a very small light spot. Information is recorded as information bit strings which can be optically detected. Information is reproduced from the recording medium by scanning information bit strings in the information tracks with a light beam spot of a certain power such that no signal is recorded on the recording medium, and by detecting reflected light or transmitted light from the medium.
As typical examples of an optical system for optically recording and reproducing information in this manner, a single light source system and a multiple light source system are known. Examples of such light source systems will be described. A single light source system will first be described with reference to FIG. 1. Referring to FIG. 1, a beam of light emitted from a semiconductor laser device 101 is made parallel by a collimator lens 102 and is thereafter split into a plural light beams by a diffraction grating 103. The split light beams travel through a polarized beam splitter 104, a 1/4 wavelength plate 105 and an objective lens 106 to be condensed on the surface of an optical card 107. Reflected light from the optical card 107 travels through the objective lens 106, the 1/4 wavelength plate and the polarized beam splitter 108 and a toric lens 108 to be incident upon a light detecting device 109. Recording, reproduction and automatic focusing control (hereinafter referred to as "AF") are performed by using 0-order diffracted light in the light beams split by the diffraction grating 103, while automatic tracking control (hereinafter referred to as "AT") is performed by using .+-.1-order diffracted light. An astigmatism method is adopted to perform AF, while a three-beam method is adopted to perform AT.
FIG. 2(a) is a schematic plan view of the optical card 107 used in the above-described information recording and reproducing apparatus. A multiplicity of information tracks for recording and reproducing information are formed on the optical card 107 so as to extend parallel to each other. Only tracks T1, T2, and T3 among them are illustrated. These information tracks are separated by tracking tracks tt1 to tt4, which are formed by grooves in the card or by a material having a reflectivity different from that of the material of the tracks T1 to T3, and which are used as guides for obtaining a tracking signal. In FIG. 2(a), an example of recording of information in the track T3 or reproduction of information from this track is illustrated. A light spot 110 of .+-.0-order diffracted light for recording, reproduction and AF irradiates the track T3 while light spots 111 and 112 of .+-.1-order diffracted light irradiate to the tracking tracks tt3 and tt4.
A later-mentioned tracking error signal is obtained from reflected light from the light spots 111 and 112, and tracking control is performed on the basis of the tracking error signal so that the light spot 110 scans correctly along the track T3. The light spots 110, 111, and 112 are moved by an unillustrated mechanism so as to scan horizontally as viewed in FIG. 2(a) while being maintained in the same positional relationship, thereby recording or reproducing information. This scanning is accomplished by a method of moving the optical system or a method of moving the optical card. In either case, the optical system and the optical card make a relative reciprocating motion and, therefore, they are not moved at a constant speed near opposite ends of the optical card. FIG. 2(b) shows the speed of this relative motion. The abscissa of FIG. 2(b) represents the distance in the horizontal direction of the optical card while the ordinate represents the scanning speed. Ordinarily, a constant speed area at the center of the optical card 107 is used as a recording area. Thus, FIG. 2(b) shows a central recording section-constant speed area, and two reversing areas at both ends of the card where the light spots reverse their direction of travel by decelerating and then accelerating. Therefore, the reversing areas comprise acceleration/deceleration areas.
FIG. 3 is an enlarged illustration of a portion of the information track T3 and portions of the adjacent tracks shown in FIG. 2(a). The light spot 110 of 0-order diffracted light for AF is positioned at the center of track T3 between the .+-.1-order diffracted light for AT and scans the center line of the track T3. Hatched areas 113a, 113b, and 113c represent digital information recorded by a 0-order diffracted light spot 110a shown in FIG. 4. Such areas are generally called information pits. The information pits 113a, 113b, and 113c have a reflectivity different from that of the track portion surrounding them. Therefore, when the information pits are again scanned with the light spot 110 at a lower intensity than that at which they were recorded, reflected light of the light spot 110 is modulated at the pits 113a, 113b, and 113c to obtain a reproduction signal in accordance with recorded information.
FIG. 4 is a schematic diagram showing details of the light detecting device 109 shown in FIG. 1 and a signal processing circuit for processing output signals from the light detecting device 109 to form a reproduction signal and a servo error signal. The light detecting device 109 is constituted by six photosensors, i.e., 4-split photosensors 114 and photosensors 115 and 116. Light spots 110a, 111a, and 112a are formed by reflected light of the light spots 110, 111, and 112 projected onto sensing surfaces of these sensors. The light spot 110a is condensed on the 4-split photosensors, while the light spots 111a and 112a are condensed on the photosensors 115 and 116, respectively. Two sets of sensor outputs in diagonal directions of the 4-split photosensors 114 are respectively added by addition circuits 117 and 118, and addition outputs from these addition circuits are further added by an addition circuit 121 to output an information reproduction signal RF. That is, the information reproduction signal RF is a signal representing the total sum of detection signals obtained as detection output fragments from the 4-split photosensors 114. The outputs from the addition circuits 117 and 118 are subtracted from each other by a differential circuit 120 to output a focusing error signal Af. That is, the focusing error signal Af is a signal representing the difference between the sums in the diagonal directions of the 4-split photosensors 114. This astigmatism method will not be described because it is described in detail in published documents and because it is not directly related to the present invention. Outputs from the photosensors 115 and 116 are subtracted from each other by a differential circuit 119 to output a tracking error signal A.sub.ts. Ordinarily, tracking control is performed in order to prevent deviation of the light spots from the information track by controlling the tracking system so that the tracking error signal A.sub.ts. becomes zero.
That is, when the portions of the light spots 111 and 112 located on the tracking tracks tt3 and tt4 have the same area, the quantities of reflected light of these light spots received by the photosensors 115 and 116 are equal to each other. Accordingly, if the apparatus is controlled so that the tracking error signal A.sub.ts representing the difference between the outputs from the photosensors 115 and 116 becomes zero, then the light spot 110 of 0-order diffracted light scans the center of the information track T3, thus normally performing tracking control. In FIG. 4, a block 122 represents an addition circuit which adds the output signals from the photosensors 115 and 116 to output a tracking sum signal A.sub.ta, as described below in detail. T.sub.s1 denotes a received light signal from the photosensor 115, while T.sub.s2 denotes a received light signal from the photosensor 116.
FIG. 5 is a diagram showing changes in the received light signals T.sub.s1 and T.sub.s2 from the photosensors 115 and 116 when the light spots 110 to 112 deviate to the left and right from the information track. The abscissa represents the deviation of the light spots from the center of the information track along the transverse direction perpendicular to the center line of the track, and the ordinate represents the quantity of light of the received light signals (light quantities) T.sub.s1 and T.sub.s2 from the photosensors 115 and 116. When the light spot 111 or 112 has no portion located on the tracking track tt3 or tt4, the received light signal T.sub.s1 or T.sub.s2 from the photosensor 115 or 116 is at a solid reflection level. When the area of the portion located on the tracking track is maximized, the received light signal is at a tracking track reflection level.
FIG. 6 is a diagram showing changes in the tracking error signal A.sub.ts when the light spots 110 to 112 deviate to the left and right from the information track. The abscissa represents the deviation of the light spots from the center of the information track along the transverse direction perpendicular to the center line of the track, and the ordinate represents the amplitude level of the voltage of the tracking error signal A.sub.ts. When the light spot 110 is positioned at the center of the information track, the light quantities T.sub.s1 and T.sub.s2 are equal to each other and the value of the tracking error signal A.sub.ts is zero. The amplitude level of the tracking error signal A.sub.ts varies in plus and minus directions according to the directions of deviation of the light spots to the left and right. If the light spots 110 to 112 deviate in a direction perpendicular to the track to such a large extent that the light spots 111 and 112 have no portions located on the tracking tracks tt3 and tt4, then the signals T.sub.s1 and T.sub.s2 become equal to each other at the solid reflection level and the tracking error signal A.sub.ts also becomes zero. This state is established when the light spot 111 or 112 is at a position X1 or X2 of FIG. 6.
Referring again to FIG. 3, if the light spot 110 of 0-order diffracted light scans along different scanning loci at the times of recording and reproduction, that is, if tracking misalignment occurs, the contrast and the pit time interval of the information reproduction signal RF may vary to such an extent that the information cannot be reproduced. Such a situation may take place due to vibration of the apparatus, or dust or a scratch on the optical card 107. If different apparatuses are respectively used for recording and reproduction, such a situation may also occur due to a difference between the characteristics of the apparatuses. In particular, in the case of a single light source system such as that illustrated in FIG. 1, there is a possibility of information reproduction failure even when tracking misalignment between recording and reproduction is small, since the size of the light spot is constant during recording and reproduction. It can therefore be said that the tracking margin in single light source systems is disadvantageously small. Moreover, the powers of the light spots 110, 111, and 112 are largely changed during recording and non-recording times, and the light spots 110a, 111a, and 112a are also changed correspondingly to affect the AF control and the AT control.
A dual light source system is known in which the recording-reproducing tracking is increased in comparison with that in the single light source system to prevent power variation in the light detecting device. Details of such a dual light source system will be described. The operation on the optical card will first be described below with reference to FIG. 7. In a dual light source system, the three light spots of the single light source system are not used for recording information; rather a recording light spot 225 is separately provided. A light spot 226 for reproduction and AF control and light spots 227 and 228 for AT control correspond to the light spots 110, 111, and 112 shown in FIG. 3. The light spots 226, 227, and 228 are equal in size but the light spot 225 is smaller than the light spots 226, 227, and 228. FIG. 7 illustrates a situation where the track T3 is scanned with the light spot 225 in the direction of the arrow to record information. The light spot 226 moves ahead of the light spot 225.
The width of a pit 229a recorded with the light spot 225 as indicated by hatching is smaller than that of the reproducing light spot 226. Therefore, even if the scanning locus of the light spot 226 is shifted from that of the light spot 225 to a small extent, the information reproduction signal RF is not considerably influenced by such a shift, in contrast with the case of the single light source system shown in FIG. 3. The tracking margin becomes larger if the ratio of the sizes of the light spot 226 and 225 is increased in this manner. However, a reduction in the contrast of the information reproduction signal RF also results and, therefore, the increase in the size of the light spot 226 must be limited. If the wavelength of the light of the light spot 225 is selected so as to be different from that of the light spots 226, 227, and 228, reflected light of the light spot 225 can easily be separated by using a dichroic mirror to be prevented from mixing in the output from the light detecting device to affect the AF and AT controls.
An example of the construction of such a dual light source system will be described with reference to FIG. 8. In the arrangement shown in FIG. 8, an astigmatism system is used for AF control and a three-beam system is used for AT control. Referring to FIG. 8, a recording semiconductor laser device 201 emits a divergent beam of laser light having a wavelength of 830 nm. The divergent beam is changed into a parallel beam by a collimator lens 203. The parallel beam travels through a dichroic prism 207, a polarized beam splitter 208 and a 1/4 wavelength plate 209 to be incident upon an objective lens 210. The beam is condensed as a small light spot on an optical card 211 by the objective lens 210 to record recording pits on a recording surface of the optical card 211 in accordance with information. The optical card 211 is the same as the optical card 107 shown in FIG. 1. Reflection light from the optical card 211 travels through the objective lens 210 and the 1/4 wavelength plate 209 and is reflected by the polarized beam splitter 208 to travel toward a light detecting device 213 (same as the device 109 shown in FIG. 1). In this case, the reflected light is reflected and absorbed by a toric lens 212 having a film capable of cutting light having a wavelength of 830 nm which is prevented from reaching the light detecting device 213, thereby preventing 830 nm light from adversely influencing the information reproducing system and the AT/AF control system.
A divergent light beam from a reproducing semiconductor laser device 202 having a wavelength of 780 nm is changed into a parallel beam by a collimator lens 204, limited by an aperture 205, and split into plural beams by a diffraction grating 206. These plural beams are reflected by the dichroic prism 207 to irradiate the optical card 211 with a small spot by traveling along an optical path which is substantially the same as the optical path from the semiconductor laser device 201. Reflected light from the optical card 211 travels through the objective lens 210 and the 1/4 wavelength plate 209, is reflected by the polarized beam splitter 208 and is condensed on the light detecting device 213 by the toric lens 212.
The light beam from the semiconductor laser device 202 forms, on the optical card 211, light spots larger than the light spot formed by the light from the semiconductor laser device 201, because of the aperture control with the aperture 205. The semiconductor laser device 202 is used for focusing control and tracking control and is therefore driven by a reproducing laser driver 223 so that the quantity of light emitted therefrom is constantly set to a small value irrespective of recording and reproduction. Information output from a controller 220 having a micro processing unit (MPU) is modulated in a modulation circuit 221 to form recording codes, and a recording laser driver 222 drives the recording semiconductor laser 201 in accordance with the recording codes to record the information on the optical card 211. Ordinarily, in actual apparatuses, recording information is supplied from an external unit. In such a case, the controller 220 includes an interface for connection with the external unit, and recording information is transmitted through the interface. Reproduced information is also transferred to the external unit through the interface.
The light detecting device 213 is the same as the light detecting device 109 shown in FIG. 1. More specifically, it includes the same elements as photosensors 114 to 116 shown in FIG. 4. A light receiving processing circuit 216 has the same components as the addition circuits 117, 118, and 121 and the differential circuits 119 and 120 shown in FIG. 4, and forms information reproduction signal RF, focusing error signal Af and tracking error signal A.sub.ts on the basis of signals of light received by the light detecting device 213. Focusing error signal Af is used to perform focusing control in such a manner that a focusing coil 214 is driven through an AF servo circuit 217 to displace the objective lens 210 in a focusing direction so that the light spots are focused on the optical card 211. Similarly, tracking error signal A.sub.ts is used to perform tracking control in such a manner that a tracking coil 215 is driven through an AT servo circuit 218 to displace the objective lens 210 in a tracking direction. The optical card 211 is reciprocatingly moved in the direction of the arrows shown in FIG. 8 relative to the light spots by a reciprocating movement mechanism (not shown), thereby scanning the information tracks of the optical card 211 with the light spots. Also in the thus-constructed dual light source system, when light spots deviate in a direction perpendicular to the track, received light signals T.sub.s1 and T.sub.s2 from the tracking control photosensors 115 and 116 of the light detecting device 213 change as shown in FIG. 5 and the tracking error signal A.sub.ts changes as shown in FIG. 6.
If a tracking control error occurs during information recording, the light spots may move to an adjacent or other track to destroy information already recorded in this track by doubly recording new information on the already-recorded information. To prevent information from being destroyed in this manner, a destruction prevention means for preventing information destruction by detecting a tracking control disturbance is adopted in each of the single and dual light source systems. In general, a tracking control error is detected through the level of the tracking error signal. That is, as explained above with reference to FIG. 8, the level of the tracking error signal is zero when the light spot 110 is positioned at the center of the information track. If the light spot 110 deviates in a direction perpendicular to the tracking direction from this state, the tracking error signal level changes in the plus or minus direction according to the direction of the deviation. A method is therefore adopted in which, when this level exceeds a predetermined positive or negative level, the occurrence of a tracking control error stops the information recording operation and prevents information from being destroyed by overwrite recording.
Such a prevention method, however, entails a problem described below. If the light spots 111 and 112 deviate from the tracking tracks tt3 and tt4 so that no portions thereof are located on the tracking tracks, then the tracking error signal becomes zero, as described above with reference to FIG. 6. This state cannot be discriminated from the normal state of tracking control. Thus, there is a possibility of failure to detect a tracking control error. This problem will be explained with reference to FIG. 9. FIG. 9 illustrates a situation where light spots 110 to 112 of the single light source system or light spots 225 to 228 of the dual light source system are scanning the information track T2 in the direction of the arrow to record information, and where a defect 224, such as a medium defect, an foreign particle attached to the card or a scratch, exists in an intermediate portion of the information track T2.
In the situation illustrated in FIG. 9, tracking control is normally made before a point O is reached. When the spots thereafter pass over the defect 224, the tracking error signal cannot be formed normally and tracking control is disturbed, so that the light spots move toward the adjacent information track T3. When the light spots pass the defect 224 and reach a point X such that no portions of the tracking control light spots 111 and 112 or 227 and 228 are located on the tracking tracks tt3 and tt4, the tracking error signal becomes zero, the tracking state is recognized as normal and this tracking control error cannot be detected. Thus, if such defect 224 exists on the information track, there is a possibility that the light spots moves to the adjacent information track T3 while the detection systems fails to perform tracking control, and new information is recorded over already-recorded information to destroy the same.